The popularity of dental resin composites as the anterior restorative material of choice has continued to grow over recent decades. Resin composites also are widely used in posterior applications, particularly in small, lightly loaded restorations.
Dental resin composites have not been universally accepted for use in large posterior restorations for a number of reasons, one being cure shrinkage. Volumetric shrinkage—defined as shrinkage of a curing resin after gelation has occurred—can generate interfacial stresses at the resin-tooth junction. These interfacial stresses can debond the dentin-restorative interface or fracture the tooth or restorative, which can cause marginal leakage, microbial attack, and long term catastrophic failure of the bulk restoration.
The remarkable advances in dentinal bonding over the past two decades have not led to consistent amalgam-like sealing of resin composite restoration margins. Instead, the gap has simply moved from the restoration interface to an adjacent area, totally within the tooth or totally within the restoration. Even if the interface remained intact, shrinkage induced internal stresses may contribute to long-term stress crack formation and even long-term failure.
Liquid crystal monomers have promise for reducing cure shrinkage and consequent stresses in dental composites. Of greatest interest are the bifunctional terminated, nematic liquid crystal monomers of the type Cn(R2,R1,R3) shown below:
R3=H or CH3
Unfortunately, the routes used to date to synthesize these LC monomers are complicated, and produce very low yields.
Also, the liquid crystal monomer should maintain a liquid crystalline state during processing in order to be useful in forming dental composites. For comfort in dental applications, the resin should be curable at ambient temperature up to body temperature, or from about 20° C. to about 37° C. Unfortunately, a resin containing only a single bisacrylate terminated, nematic liquid crystal monomer has been found to rapidly crystallize upon addition of filler. It may be possible to alleviate the problem of premature crystallization by blending of monomers; however, certain blends exhibit much higher polymerization shrinkage than others.
Liquid crystal monomers also present a problem because cure shrinkage is temperature dependent, and increases rapidly as the N→I transition temperature is approached. If the N→I transition temperature of the LC monomer resin is just slightly above mouth temperature, then excessive polymerization shrinkage may result.
Economic methods are needed for making liquid crystal monomers on a commercial scale, and for producing blends of liquid crystal monomers that both remain in a nematic state under processing conditions and exhibit extremely low cure shrinkage.